1. Field of the Invention
The present invention concerns a method for error-tolerant data transfer between a data acquisition unit and an image reconstruction device of a CT system of the type wherein digital data are divided into data packets and are transferred as a data stream from a transmission device to a reception device and are buffered (cached) in the reception device for further processing. The invention also concerns a computed tomography (CT) system with such an apparatus for data transfer between a data acquisition unit and an image reconstruction device.
2. Description of the Prior Art
Error-free data transfer is important in many technical fields. In the field of imaging, particularly medical imaging, optimal error-free transfer of the measurement data from the data acquisition unit at the measurement location to the image reconstruction unit is an important requirement for achieving a high-quality image reconstruction. The problem of transfer errors in particular occurs with wireless transfer techniques as used, for example, in computed tomography systems. Data transfer ensues in CT systems via an optical or capacitive slip ring between the rotating part and the stationary part in order to be able to transfer measurement data acquired by the data acquisition unit at the measurement location to the stationary image reconstruction device during the measurement. This data transfer technique in CT systems, however, is sensitive to external interferences that can cause incorrect data bits in the data transfer. While these bit errors frequently can be corrected using suitable error correction techniques, but an interruption of the data transfer or an occasional missing synchronization between the transmission device and the reception device lead to larger problems. Such interruptions occur with higher probability in multi-slice CT systems because external interferences influence a larger number of data bits due to the very high transfer rate in these systems within the same time duration as in other systems. In current multi-slice CT systems, the measurement data are transferred in a serial bit stream in which the serial bit clock is embedded. The reception device extracts the embedded clock in order to decode the bit stream. Furthermore, special serial codes are periodically transferred with the bit stream in order to achieve a byte and packet synchronization. An external interference can lead to an error in the transfer of these synchronization codes that can result in a desynchronization. In this case, transferred data packets are not received or decoded or are only incompletely received or decoded, such that they are missing for the subsequent image reconstruction.
Due to missing data packets in the transmission of the measurement data in a computed tomography system, it may be necessary to interrupt the entire measurement process because a correct image reconstruction is no longer possible due to the missing data. This has as a consequence a loss of expensive contrast agent (if used in the examination), an increased patent radiation exposure and additional time expenditure.
A method for error-tolerant data transfer is known from U.S. Pat. Application Publication No. 2003/0185427, in which incorrect data bits in the transferred data packets are replaced by bit values that are interpolated from adjacent data packets. The incorrect data packets can be identified by an error checksum transferred with the data packets.
Furthermore, in a technique known as FEC (Forward Error Correction), additional coding bits, from which the correct values of incorrect data bits can be reconstructed, are transferred together with the data.
U.S. Pat. Application Publication No. 2003/0229840 concerns a method for forward correction (FEC) in the transfer of data packets over a network. In this method, additional parity packets, from which later missing data packets can be recovered, are generated from the data packets to be transferred. These parity packets must be transferred over the network in addition to the data packets and thus reduce the data transfer rate. Use of this technique in CT systems therefore is not advantageous. Missing data packets normally are recovered from the information in the parity packets. Given unrecoverable packets, it is proposed either to replace these with predetermined replacement packets or to form an average value from the data of a number of preceding and subsequent data packets, and to replace the missing data packet with an average data packet so obtained. These method steps ensue in the JPEC decoder of the reception device discussed in that published application.
U.S. Pat. Application Publication No. 2002/174403 also concerns a method for data transfer in a network in which additional parity packets are generated and transferred. The transfer ensues in a multidimensional matrix that is intended to make the later recovery of lost data packets via the parity packets easier. Missing data packets are represented with a placeholder upon receipt of the matrix and are subsequently reconstructed using the parity packets. Missing data packets, for which a recovery via the parity packets is not possible, are not replaced.
PCT Application WO 01/28252 discloses a method for image coding in which the data transfer ensues with redundant block code and auxiliary data that are progressively coded for each image. It is thus a method for transfer of images, rather than measurement data or raw data from which corresponding image data then can be generated in the image processing unit of a CT system downstream from the reception device.
The abstract of Japanese Application 01221958 discloses an interpolation circuit for an incorrect data packet. In this circuit, two separate reception buffers are provided, of which one serves for acquisition of normal data packets and the other serves for acquisition of defective data packets or data packets to be interpolated.